Present Floodplain Research Articles

Orbital global change drove fluvial aggradation and incision in Tibetan upper Mekong river: Chronological perspectives

The Tibetan Plateau (TP), often known as the “Asia Water Tower”, is the source region of several continental-scale rivers. However, due to the extremely difficult access and harsh living conditions in the interior plateau, fluvial processes in the headwaters of these large rivers and their connections to global climate changes remain unknown. In this study, the luminescence (using both quartz and K-feldspar) and AMS 14C dating techniques were employed to date the four terraces of the Zado Basin in order to elucidate the aggradation and incision history of the initial intermontane basin in the headwater of the Mekong River. One sample from the upper part of T4 terrace (which features a 12 m thick gravel layer, the base of which is unexposed) provides a K-feldspar pIR50IR225 age of 125 ± 10 ka, indicating that the initial deposition of this terrace could predate marine isotope stage (MIS) 5. Considering the margin of error, this age also suggests that the onset of incision on T4 terrace during the climate transition from MIS 6 (glacial) to early MIS 5 (interglacial). This incision likely occurred due to increased meltwater and precipitation, causing the Mekong carving into the bedrock for more than 50 m. Eight luminescence samples are collected from T3 terrace (26.5 m in thickness) with ages ranging from 83 ± 7.6 ka to 22.3 ± 1 0.2 ka, indicating T3 formation/aggradation from late MIS 5 to the Last Glacial Maximum (LGM), possibly owing to reduced precipitation and enhanced sediment supply from intensive glacial activities. At around 22 ka, the Mekong once again shifted towards incision, crafting the T3 terrace, situated roughly 28 m above the current floodplain. This incision event was recorded by two luminescence samples from a sandy lens located at the top of gravel layer of T3, with OSL ages of about 22 ka. Floodplain samples from T2 and T1, dated at 16 ka and 2.8 ka respectively, demonstrate that from the last deglaciation to the Holocene, the Mekong continued its downward incision. This erosion developed two cut and fill terraces at elevations of 22 m and 11 m above the present floodplain, likely due to increased precipitation and enhanced vegetation at that time. Overall, the glacial–interglacial climate changes were the most likely driving force behind fluvial processes in the upper Mekong basin since the Late Pleistocene, with aggradation occurring during the glacial period while incision in deglacial and interglacial periods, suggesting a close link between the fluvial geomorphic evolution and orbital-scale climate changes.

Geochemistry and mineralogy of modern floodplain Nile sediments of Sohag area, Egypt: an environmental perspective

Understanding the characteristics of modern floodplain sediment (MFS) is important for geomorphological, paleoclimatic, sedimentation, and environmental investigations. We collected samples from present floodplain Nile sediments in Egypt’s Sohag area and used mineralogical and geochemical (i.e., major and trace elements) proxies to investigate the characteristics and establish their provenances, climatic conditions during deposition, and weathering influence. The study revealed that the Nile floodplain sediments are composed mainly of silt (26–77%), clay fraction (7–44%), and sand content (3–63%), and their texture is classified as clayey silt. These sediments are classified geochemically as greywacke to shale. The mineralogical studies revealed two main assemblages of heavy minerals: the first assemblage represented by magnetite, goethite, zircon, epidote, and garnet and reflects the basement complex in the Eastern Desert and its surrounding rocks; and the second assemblage of goethite, pyroxene, hornblend, and mica of fooldplain deposits derived from the Ethiopian highlands. The geochemical studies indicate that the Nile sediments are immature and formed due to the moderate degree and nonsteady chemical weathering of the parent rocks. The geochemical studies show different mixed sources of sediments, where fine sediments are mostly mafic igneous with a partial contribution from siliscic sedimentary rocks, while the coarser ones are derived mainly from siliscic sedimentary and acidic igneous rocks. The sediments of the presented modern fooldplain reflected two different tectonic setting source areas (collision and continental rift tectonic setting for high silica and low silica samples, respectively) that were shared in supplying the sediments to the present basin. The study revealed that the investigated area may become desertified as a result of climate change and human activities such as salinity, fertiliser use, sand encroachment, and pollution.

Late Quaternary aggradation and incision in the headwaters of the Yangtze River, eastern Tibetan Plateau, China

AbstractRiver aggradation or incision at different spatial-temporal scales are governed by tectonics, climate change, and surface processes which all adjust the ratio of sediment load to transport capacity of a channel. But how the river responds to differential tectonic and extreme climate events in a catchment is still poorly understood. Here, we address this issue by reconstructing the distribution, ages, and sedimentary process of fluvial terraces in a tectonically active area and monsoonal environment in the headwaters of the Yangtze River in the eastern Tibetan Plateau, China. Field observations, topographic analyses, and optically stimulated luminescence dating reveal a remarkable fluvial aggradation, followed by terrace formations at elevations of 55–62 m (T7), 42–46 m (T6), 38 m (T5), 22–36 m (T4), 18 m (T3), 12–16 m (T2), and 2–6 m (T1) above the present floodplain. Gravelly fluvial accumulation more than 62 m thick has been dated prior to 24–19 ka. It is regarded as a response to cold climate during the last glacial maximum. Subsequently, the strong monsoon precipitation contributed to cycles of rapid incision and lateral erosion, expressed as cut-in-fill terraces. The correlation of terraces suggests that specific tectonic activity controls the spatial scale and geomorphic characteristics of the terraces, while climate fluctuations determine the valley filling, river incision and terrace formation. Debris and colluvial sediments are frequently interbedded in fluvial sediment sequences, illustrating the episodic, short-timescale blocking of the channel ca. 20 ka. This indicates the potential impact of extreme events on geomorphic evolution in rugged terrain.

Quaternary evolution of the Ploučnice River system (Bohemian Massif) based on fluvial deposits dated with optically stimulated luminescence and in situ produced cosmogenic nuclides

The Ploučnice River system, located in the central Bohemian Massif, is draining an area almost not covered by continental ice sheets but instead archiving the fluvial deposits. The fluvial style changes from a high-energy braided to a long-bend meandering river in the upper terrace levels (36 to 29 m above present floodplain). The middle terrace levels (22 to 14 m above present floodplain) indicate a fluvial style changing from a high- to medium-energy braided river. In the lower terrace levels (13 to 5 m above present floodplain), the terrace deposits indicate high-energy braided to long-bend meandering river environments. To provide greater details on the timing of fluvial terrace formation, this study applied 26Al and 10Be isochron burial and optically stimulated luminescence (OSL) dating methods to terraces of the Ploučnice River system. Terrace fluvial infills are dated with isochron burial dating in relative altitude 34, 29, and 14 m above present floodplain (apf); whereas terrace accumulations in relative altitude 19, 12, and 6 m apf are dated with OSL. Because of differences in age results between the two dating methods, we propose two different evolution models: the first is based on isochron burial and OSL dating, and the second model is on OSL dating results only. The time span represented by the river terraces remains unclear and varies from Eburonian to Weichselian (1680 to 56 ka) or from Saalian to Weichselian (138 to 56 ka) with the estimation of the highest terrace (248 ka) respectively. The former river evolution model is based on tectonic activity at least since 1000 ka. Morphotectonic analysis recognized new lineaments of which the general direction corresponds with the main direction of the Ohře fault zone (NE- to ENE-striking) and Lužice fault zone (NW-striking). Based on dated terrace ages of 1153 ± 123 ka at 14 m apf and 138 ± 21 ka at 19 m above present floodplain, we propose a normal fault being active from at least 1153 ka. The second river evolution model assumes possible remobilization of clasts analyzed by isochron burial dating before their final deposition. From three OSL ages we calculated a mean incision rate and estimated an age of upper terrace levels at 34 m apf to be 248 ± 38 ka (Saalian age). As remobilization of clasts in high-energy fluvial and glaciofluvial environments is very likely, age determination is challenging. Nevertheless, we interpret the terrace record in the Ploučnice River system as a product of Quaternary climatic changes influenced by tectonic processes.

Discriminating between the roles of late Pleistocene palaeodischarge and geological‐topographic inheritance in fluvial longitudinal profile and channel development

AbstractThis paper investigates the influences of palaeohydrology and geological‐topographic inheritance in shaping the channel of the lower River Suir, southeast Ireland. Results of acoustic surveys of the lower River Suir and Waterford Harbour reveal two scales of pseudo‐cyclic river bedforms. Longitudinal elevation profiles of the geological topography (undulating bedrock and till‐mantled bedrock) bounding the present floodplain swath reveal pseudo‐cyclicity in that terrain too. Spectral and statistical analyses are used to quantify the cyclicity of the long profile and geological‐topographic series. These methods show that the dominant cyclicity of the long profile reflects autocorrelation more than inheritance of cyclicity from the bounding geological topography. The cyclicity of the long profile mainly reflects a hydraulic control on pool‐spacing, although some cyclicity probably has been inherited from the geological‐topography. Channel‐forming palaeodischarge is estimated based on the dominant pool‐spacing revealed by spectral analysis, validated using relationships between meander wavelength, channel cross‐sectional geometry and hydraulically‐informed discharge reconstruction. The palaeodischarge estimates are in close agreement and are two orders of magnitude greater than present flood maxima. Significantly, these palaeodischarge estimates also agree closely with palaeodischarge calculated for the submerged Pleistocene palaeochannel that extends across the near‐shore continental shelf from Waterford Harbour. The pool‐sequence of the lower Suir and the submerged palaeochannel represent a former land‐system that was active during a period of low relative sea level during the last glacial. More broadly, the paper offers insights into the landscape evolution of formerly glaciated regions that experienced very wide discharge variability during and after the transition from glacial to interglacial regimes, in a context of complex relative sea level change. Copyright © 2017 John Wiley & Sons, Ltd.

Late- and Post-glacial history of the East Branch of the Penobscot River, Maine, USA

Between ~20 and 15 ka the Laurentide Ice Sheet retreated from the edge of the continental shelf, ἀrst to the Maine coast and then across Maine to the northern reaches of the Penobscot Lowland. The Lowland, being isostatically depressed, was inundated by the sea. As ice then retreated into Maine’s western mountains, valleys through the mountains became estuaries. In the estuary now occupied by the Penobscot River’s East Branch, ten ice-marginal deltas were built during pauses in this part of the retreat. By 14 ka the ice had retreated far enough to expose land in the valley bottom between the ice front and the sea, and the Penobscot River was (re)born. This occurred near the present conᴀuence of the Seboeis River and the East Branch. The river gradually extended itself northward as the ice retreated and southward as relative sea level fell. Braidplains were formed and incised, leaving terraces. High initial discharges eroded the eastern ᴀanks of the esker and deltas, redepositing silt, sand, and gravel all the way to the present head of Penobscot Bay. By ~10 ka the discharge had decreased, the river was adjusting to on-going differential isostatic rebound, and finer sediment was accumulating, forming the present floodplain.

HISTORY OF THE LOWER VOLGA PALEO-DELTAS FORMATION

The Paleo-Volga River valley existed within the present Lower Volga basin during the last 600-700 ka. Periodically its lower parts were transformed into a long and deep ingressive estuary. Over the last 16 ka there have been 6 marine (estuarine) phases within the Volga-Akhtuba valley corresponding to transgressive phases of the Late Khvalyn and New Caspian ages. There are pronounced traces of the 4 transgressive-regressive stages of the Late Khvalyn and New Caspian ages in the present Volga-Akhtuba floodplain topography, correlated with the three generations of ancient floodplains, whose age are decreasing downstream from 16 to 1.5 ka.

Deep gullies entrenchment in valley fills during the Late Holocene in the Basento basin, Basilicata (southern Italy)

Deeply entrenched channels namely badland gullies occur on the gently sloping valley floors of any alluvial basins in the semi-arid environments of Fossa Bradanica, Basilicata, southern Italy. Valley fills in this region consist of predominantly Holocene alluvium; late Pleistocene deposits occur as terraces above the present flood plain or as eroded benches buried beneath younger alluvium. Historical fluvial deposition began before 7000 a cal. BP and ended at around 120 a cal. BP. In this study, we focus on the channel entrenchment (badland gullies incision) in the last 4500 years, based on a detailed study of chronostratigraphic alluvial sequences of Basento River. Four main phases of incision have been recorded at around 4500-4350, 1500, 900-800, and 120 a cal. BP. The formation of late-Holocene badland gullies can be related to changes in climate, vegetation and groundwater conditions. Environmental history mainly derived from the high-resolution pollen record of Lago Grande di Monticchio. Badland gullies formation appears to be linked to repeated dry-wet cycles. During dry periods, water tables significantly dropped and vegetation cover decreased with increase in runoff and water discharge from rainfall, leading to deepening and widening of stream channels.

Transient Two-Dimensional Simulation of Real Flood Events in a Mediterranean Floodplain

The application of a two-dimensional (2D) finite volume numerical model to real flood events in the Ebro River is presented. The hydraulic model used is based on the 2D transient shallow-water equations on the irregular bed that are able to compute flow advance over a dry bed. This study involves the reliable simulation of not only the flood wave advance but also the drying process in a series of events of different magnitude. The importance of the correct characterization of the roughness coefficient and the topography is emphasized in the study. The former is estimated from a previous classification of structurally homogeneous habitats, and the latter is defined by merging the digital terrain model data with a hydraulic river bed elevation reconstruction algorithm. The calibration of the full model resulting from the roughness, bed river, and flow simulation models is based on field measurements of the flooded area for two steady discharges. The validation is performed by comparing the numerical results with the water levels measured during five flood events at certain times, with the flooded area and time series of continuous point measurements of water depth during different situations throughout the year. Because the model provides correct predictions of the surface processes both for low and high flow discharges, the simulation results are used to analyze the present floodplain hydrodynamics. In the same way, different topographic scenarios, on the basis of changes in the hydraulic river- floodplain connectivity, are generated to analyze their potentially beneficial effect in the floodplain geomorphic dynamics. DOI: 10 .1061/(ASCE)HY.1943-7900.0000565. © 2012 American Society of Civil Engineers. CE Database subject headings: Unsteady flow; Overland flow; Hydration; Dewatering; Flood plains; Shallow water; Mediterranean Sea; Simulation. Author keywords: Unsteady surface flow; Wetting/drying; Finite volume; Shallow water; Hydrodynamic; Floodplain; Hydraulic bed reconstruction.

Distribución y evolución histórica de las terrazas fluviales del valle de Santa Teresa Guanajuato

Este trabajo reporta la existencia de terrazas fluviales originadas por eventos de inundación históricos a lo largo del valle fluvial de Santa Teresa en el Río Guanajuato. Se identificaron y cartografiaron seis diferentes niveles de terrazas que corresponden a T1 (1.8 m), T2 (3.0 m), T3 (4.2 m), T4 (5.2 m), T5 (6.0 m), T6 (+ 6.8 m).Con base en el análisis de secuencia de terrazas y apoyados con dos dataciones isotópicas de C14 , se comprobó que la T1 corresponde a la planicie de inundación máxima actual, las terrazas T2, T3, T4 se formaron en períodos de avenidas extraordinarias, con edades de entre 10 a 100 años en relación con el presente. La morfología actual de T5 corresponde a un evento de inundación histórica ocurrido en 1905. El proceso inicial de depósito de la terraza T6 fue seguido de un proceso de erosión que se interrumpe por un período de intenso aporte de sedimentos entre los años 1660 +/-40 a 1905 (T5) y que corresponde a la época de gran actividad minera en el distrito minero de Guanajuato. Después de 1905 el aporte de sedimentos a la cuenca disminuye sustancialmente, generando una erosión gradual que por pulsos repetitivos desciende conformando las terrazas T4 a T1.

Geochronological reconstruction of the Pleistocene evolution of the Sarre valley (France and Germany) using OSL and ESR dating techniques

This paper focuses upon the Pleistocene terraces of the Sarre River, a right bank tributary of the Moselle River (NE France and SW Germany) flowing through the Vosges Massif, the eastern Paris Basin and the Rhenish Massif. Recent research has allowed the recognition of 12 well preserved alluvial terraces (Sa1 youngest to Sa12 oldest) between the present floodplain Sa0 and +120m relative height. The youngest terraces were dated using Optically Stimulated Luminescence (OSL). A first chronological framework was also provided for older terraces by Electron-Spin-Resonance (ESR) dating. The ages range between the end of the Early Pleistocene and the last glacial–interglacial cycle of the Late Pleistocene (ca. 1.1Ma to 50ka). Age ranges are consistent with the terrace elevation and stratigraphy, allowing correlation of the youngest terraces with established global climate cycles and with the younger terraces in the Moselle valley. In particular, an erosional period was recognised at the end of the Saalian, (end of MIS 6) suggesting that the terrace incision occurred at the cold-to-warm transition. This result contrasts with those obtained for the Moselle and Meurthe Rivers, where previous studies suggest that major incision took place at the beginning of the cold periods. The differences are attributed to a variable fluvial response to climate change which could relate to the presence or absence of glaciers in the upper catchment.

REMOTE SENSING DETECTION OF DYNAMIC VARIATION OF THE SALINE LAND IN THE YELLOW RIVER DELTA

Land salinization is a dynamic phenomenon or process. With the annual and seasonal variation of the groundwater table, the nature, extension, magnitude and spatial distribution of the saline land are changing from time to time. The grasp of the up-to-date and reliable information on the saline land is a prerequisite for land reclamation and regional sustainable development. For the purpose of providing a generalized view of a fairly large area, remote sensing is widely used in saline land survey and dynamic study. Three groups of Landsat data were selected for the study of saline land dynamics during the past 15 years in the Yellow River Delta. Based on repeated field survey and soil sample analysis, an integrated classification method was developed for the extraction of information about saline lands. The study shows that, from 1987 to 2000, the trend of land salinization was rapidly accelerated: light saline land greatly increased; strong saline land gradually decreased; barren saline land increased from 1987 to 1996 and then decreased. The spatial structure of saline land distribution in such 6 types of landforms as terrace uplands, present flood plain, abandoned river courses, embanked former back swamps, isolated depressions and salt marshes tidal flats in 2000 was almost the same as that in 1987.

A suburban inter-dike river reach of a large river: Modern morphological and sedimentary changes (the Bratislava reach of the Danube River, Slovakia)

The paper outlines contemporary geomorphic and sedimentary changes (1949–2007) in the suburban inter-dike reach of the Danube River in Bratislava underpinning requirements of its management. The studied river reach represents the right bank inter-dike area of a tectonically determined bend of the Danube River which is about 5 km long with a radius of 1.5 km. The landform and floodplain roughness classification schemes show variations in hydraulic and depositional conditions during flooding. Investigation of landform and floodplain roughness changes and the bank retreat rate is based on the multitemporal interpretation of aerial photographs. Sedimentological analysis of 10 borings and 20 pit exposures helped to delimit the morphostratigraphic units. The process and the rate of the vertical accretion of the modern floodplain were investigated by sedimentological and dendrochronological methods. The behaviour of the study reach is dependent on upstream dams in Austria. Dams produce changes in the suspended load regime and a steady riverbed erosion of 2 to 3.5 cm occurs upstream of the study river reach near the Slovak–Austrian border. The Čunovo dam downstream expediates upstream channel filling in spite of the ongoing channel gravel mining in the Danube channel. This has resulted in channel bed aggradation of about 1 m since 1992. The bank retreat of about 100 m during 1949–2007 was caused by the natural bank erosion processes as well as by channel straightening as a flood control measure for Bratislava. The bank shift resulted in the development of the new levee. The current overbank deposits differ from the older gravels, consisting of a fine-grained alluvium varying in thickness from 0.5 m to 1 m. The lithofacies of three flood deposits (March 2002, August 2002, and September 2007) were examined. Overbank sediments differentiate with distance from the bank and vertically. Suburban settings of the study reach and the flood control measures along with the external forcing (dams) induced changes in channel morphology as well as in floodplain roughness variability. This has resulted in changes in the present floodplain sedimentary conditions, channel forms and biodiversity. The landscape evolved through three distinct evolutionary phases.

Late Pleistocene genesis of the middle Yarlung Zhangbo Valley, southern Tibet (China), as deduced by sedimentological and luminescence data

Knowledge on valley formation and palaeohydrology of the Yarlung Zhangbo (named Brahmaputra south of the Himalayas) is still in an early stage. Research was conducted in the middle valley reach around Gonggar in order to investigate the sediment properties, age and palaeoenvironmental implication of a widespread aggradational valley terrace. This heavily dissected terrace, lying c. 25 m above the present floodplain, consists of topping aeolian and colluvial deposits, intermediate lacustrine fines and basal fluvial sands. Sedimentological properties of the lacustrine layers suggest a cold-climate depositional environment. Geochemical data refer to a common provenance of all sediments investigated from a regional catchment area, implying a local provenance of topping loesses and sands by aeolian sorting of nearby fluvial deposits. OSL dates, representing the first luminescence dating effort partly applied to non-aeolian sediments in this valley, cluster closely in the interval 11.7 ± 0.8–17.4 ± 1.4 ka. During the Late Pleistocene (until c. 16 ka) the investigated middle valley reach around Gonggar was occupied by a lake attaining an extension of at least 100 km.

Clay coatings from a modern soil chronosequence: A tool for estimating the relative age of well-drained paleosols

Blue light optically stimulated luminescence (OSL) dates obtained from upland and terrace soil profiles in southeastern Mississippi have been used to generate a model for illuvial clay accumulation in deeply weathered, subtropical soils (e.g. Ultisols). The illuvial clays developed in upland vs. alluvial terrace soils of southeastern Mississippi are distinct in terms of thickness and total volume of translocated clay. The coatings in the Mississippi soils are also an analogue for clay coatings observed in mid-Cretaceous paleosols of North America. Many of the Cretaceous paleosols had a polygenetic history. The relative age of the well-drained phase of pedogenic development, characterized by illuvial clay accumulations, may be estimated using the model generated from the Mississippi soils. As many of the mid-Cretaceous paleosols mark unconformities, it would be beneficial to estimate the duration of the hiatuses. Three geomorphic surfaces characterize the landscape of southeastern Mississippi: uplands, stream-cut terraces, and modern alluvium. The present floodplain, has poorly developed soils, and lacks illuvial accumulations. Soils developing in the uplands have abundant and prominent illuvial clay coatings up to 2 mm thick, while the terraces have minor clay coatings less than 1 mm thick.

Fluvial evolution of the lower Mississippi River valley during the last 100 k.y. glacial cycle: Response to glaciation and sea-level change

The lower Mississippi valley contains multiple large braid belts for which age control has been limited. Application of the optically stimulated luminescence technique has produced a new chronology of lower Mississippi valley channel-belt formation and insight into the valley9s evolution during the last glacial cycle. Fluvial deposits range from last interglacial meander belts (85 ± 7 to 83 ± 7 ka) to multiple braid belts (64 ± 5 to 11 ± 1 ka) and record large-amplitude responses of the Mississippi River to glacially induced changes in discharge and sediment supply during the last glacial cycle. Slackwater deposits in buried tributary valleys from the middle Mississippi valley and northern lower Mississippi valley suggest that the river was flowing at a position 8–21 m below the present flood plain during the last interglacial, then rapidly aggraded and switched to a braided regime to form the highest and oldest braid belt by 64 ± 5 to 50 ± 4 ka, coincident with initial glaciation of the upper drainage basin. The Mississippi River remained braided until final meltwater withdrawal from its headwaters in the earliest Holocene. Braid-belt formation and incision was controlled by fluctuations in meltwater and sediment discharge, while glacio-eustatic sea level controlled the elevation to which the river was graded, causing late glacial braid belts to dip below the Holocene flood plain in the southern lower Mississippi valley. Moreover, avulsions in the middle Mississippi valley and northern lower Mississippi valley during the last glaciation have pinned the river over regions of shallow bedrock, preventing the modern river from incising to its last interglacial profile. The new chronology and longitudinal profiles presented here provide insight into the response of this continental-scale river system to climatic (glacial) and base-level forcing during the last 100 k.y. glacial cycle.

The Geomorphology of Valley-Floor Troughs Illustrated From Depositional Sequences in the Durance Valley, Southeast France

Three kinds of valley-floor troughs occur in the Durance catchment of southeast France. Details of widths and spatial variations of 10 different sedimentary forms are presented to show how these relate to the present types of troughs and their margins. Internal controls, such as geology, altitude, and available relief, provide a wide variety of conditions, which have been modified through time by external controls such as past and present climates and their hydrological regimes, as well as by anthropogenic activity over several millennia. The 10 sedimentary forms occupy more than 900 km2 or 6.4% of the total catchment area, with the present enlarged floodplain having just over 50% of this area. The present channel occupies less than 10%, which is less than the Würm terrace (17.2%) but somewhat more than the marginal colluvia in the lower valley (8.6%). The other forms include older Pleistocene terraces, moraines from the last three glacial stages, and alluvial fans. It is the type of trough and its relative age that condition what, where, and how much of past and present sedimentary forms can survive.

Evolution of the morphology of the river Dragonja (SW Slovenia) due to land-use changes

The effects of increasing agricultural land use on fluvial morphology have received much attention in fluvial research. However, in several regions in Europe, a reversing trend of decreasing agricultural activity and land abandonment, followed by reforestation, is observed. The response of fluvial morphology deserves attention because of its large impacts on landscape and riverine habitats. With the help of geomorphological mapping, multi-date aerial photography and a range of dating techniques, we reconstructed the evolution of the morphology of the riverbed and the floodplain of the Dragonja river in southwestern Slovenia. The results of this study show that the fluvial morphology in this Mediterranean catchment has changed considerably as a result of shifts in agricultural land use, in particular large-scale land abandonment in the second half of the 20th century. Until the first half of the 19th century, floodplain aggradation prevailed. Probably around 1870, a large erosion event occurred from which the floodplain did not fully recover. A terrace standing 2.5 m above the present floodplain was formed. Natural reforestation, due to depopulation since World War II, caused a reduction in discharge and sediment supply to the river. The decreased intensity and frequency of floods allowed invasion of the riverbed by vegetation, causing narrowing and incision of the riverbed. This resulted in the formation of a terrace, which now stands 1.5 m above the present-day river. This terrace is about 60 years old. However, the largest increase in forest area occurred since 1975, which intensified this process of riverbed narrowing and incision, creating a local terrace at 0.5 m at 0.5 m above the presently meandering river.

Geomorphologic evidence of phased uplift of the northeastern Qing-hai-Tibet Plateau since 14 million years ago

A typical sequence of fluvial terraces and aeolian deposits overlying these ter- races were multidisciplinary investigated. New evidences for uplift process of the northeastern Qinghai-Tibetan Plateau in the past 14 million years were obtained. At least 11 river terraces along Huangshui, the first-class tributary of Yellow River, at the Xining-Huzhu region are identi- fied. While the first one (T1) is classified as an accumulation terrace, the others are all basement river terraces, which consist of the Tertiary sandstone and siltstone bedrock, fluvial gravel and pebbles and the overlying aeolian loess-Red Clay deposit. Samples from the aeolian deposits were examined for paleomagnetic stratigraphic reconstruction (1030 samples), luminescence dating (16 samples), magnetic susceptibility and grain-size distribution (more than 4000 sam- ples). The luminescence dating and stratigraphic correlation suggest that terraces of Tll, T10, T8, T7, T3, T2, T1 were formed at 14, 11.3, 1.55, 1.2, 0.15, 0.07 and 0.01 million years ago, re- spectively. Sedimentological analysis and geomorphological observation indicate that formation and evolution of these terraces were mainly driven by tectonic uplift. Therefore, the terrace se- quence provides an ideal geological record, of the uplift process of the northeastern during the past 14 million years, and the timings of the terraces formation are regarded as the timings of tectonic uplift. The significant uplifting events took place at 14, 11.3, 1.2 and 0.15 mil- lion years ago, respectively. The fluvial incision at the Xining-Huzhu region is less than 100 m during a period of ~12 million years in the Miocene era (between the Tll and T9), while the Huangshui River had incised 432 m during the past 1.2 million years (from T7 to the present floodplain). The river incision process clearly demonstrates that accelerated rising of the north- eastern Qinghai-Tibet Plateau during the late Cenozoic, and provides new evidence of previous thoughts. There was a significant readjustment of the fluvial catchment during 1.55--1.2 million years ago: before this time, the paleoriver flowed to southwest. After this time the Huangshui River flows to southeast. A tectonic movement dominates reorganization of this fluvial system.

An evaluation of the benefits of lowland river-floodplain rehabilitation (the Rhine, the Netherlands).

Water quality of the Rhine has improved significantly since 1970, but its landscape was highly altered with a shortage of dynamic processes, characteristic landscape elements and biodiversity. During the last decade manifold floodplain rehabilitation projects have been planned and realized in the Netherlands. Yardsticks for recovery are naturalness, biodiversity and spatial connectivity, but actually there is no clear end-point or reference to assess its success. Constraints for rehabilitation are flood protection and shipping. The benefits of a subset of rehabilitation projects situated along the largest flee-flowing Rhine branch, that has an open connection to the North Sea, were assessed at two landscape levels: the project itself (1-10 kin) and the river stretch (> 100 km) using five parameters (landscape mosaic, vegetation, macro-invertebrates, fish and birds). The assessment yielded comparable marks among the parameters, so the overall appraisal is considered to be robust. However, the scarcity and incompleteness of the underlying data forced us to use a fair amount of expert judgement. Generally terrestrialized floodplains have been set back to more frequently flooded (semi-) aquatic habitats. Especially dynamic, shallow water habitats emerged. Vegetation quickly responded when management changed from farming to extensive grazing. Rheophilic fish and macro-invertebrates immediately colonised side channels. At present, more than 40 % of the present floodplain area consists fo deep water and 30 % of agricultural land and built up areas, and typical floodplain habitats (such as shallow riverbed, river banks and dunes and herbaceous floodplains) are still underrepresented. Within rehabilitation projects the landscape significantly changed, but the projects do not yet significantly contribute to the ecological recovery of the whole river stretch. Moreover, biological indicators point to a moderate or even poor ecological state.